This invention relates to a spring and in particular, though not exclusively, to an elastomeric spring of a kind which is intended normally to withstand compression load in a first direction between two opposed end regions of the spring whilst permitting a degree of substantially free relative movement between said end regions of the spring in a plane which is generally transverse relative to said first direction.
One known spring of the aforementioned kind is described in the specification of GB-A-2156947.
The ability of a spring to permit substantially free relative movement in a transverse direction usually can be improved by increasing the length of the spring in the first direction, but that requires an increase in the overall space envelope and may not be acceptable. Also the weight of the spring and cost of materials is increased.
The present invention seeks in particular, though not exclusively, to provide a spring which, in comparison with a conventional spring, has an improved ability to permit substantially free relative movement in a transverse direction without any substantial increase in the length of the spring in the direction in which normally it is loaded in compression.
In accordance with the present invention, an elastomeric spring comprises a body of elastomeric material which extends between a pair of opposed end regions, each end region being bonded to a respective one of a pair of rigid end members whereby compression forces acting in a first direction on the rigid end members are resisted by compression of the elastomeric body, the body of elastomeric material being of a substantially symmetrical shape as considered in cross-section about a major axis of the spring extending in the first direction, the body of elastomeric material being substantially hollow in the vicinity of at least one of the end regions, the body of elastomeric material being of progressively increasing external cross-sectional dimension over a first part of the length of the body extending from a first of the end regions, the body comprising a second part which extends to a second of the end regions and the external profile of the body comprising two portions of convex shape as considered in a plane which contains the major axis of the body, a substantially inextensible reinforcement member embedded in and bonded to the body of elastomeric material at a position between said first and second parts of the length of the body and extending radially outwards from the bonded position, and the body of elastomeric material being of a progressively increasing external cross-sectional dimension over a part, hereinafter referred to as a fourth part, of the length of the body extending from the reinforcement member to the second part, the body of elastomeric material being hollow over a distance which extends, in the first direction, from at least one of the end regions to beyond the position at which the reinforcement member lies between the end regions, that part of the body of elastomeric material which lies between the second end region and the reinforcement member having a maximum external dimension which is greater than that of the part of the body between the reinforcement member and the first end region, and the maximum external dimension of the bonded area of the first end region being less than the maximum external dimension of the bonded area of the second end region.
Preferably the substantially inextensible reinforcement member does not form a complete division between elastomeric material of the first and second end regions. Therefore the body of elastomeric material may extend continuously between the end regions.
The reinforcement member may be annular. The body of elastomeric material may be of circular shape in cross-section and the inner boundary of the reinforcement member may have a diameter greater than the inner diameter of the elastomeric body at a transverse plane containing the reinforcement member so that the body surface which defines the hollow is not interrupted by the reinforcement member.
The reinforcement member may be planar, and may be constructed from sheet material such as sheet metal.
The second part of the length of the body optionally may be of progressively decreasing external cross-sectional dimension in a direction from said fourth part to said second of said end regions.
The body of elastomeric material may have a profile which, as considered in a longitudinal cross-section, in a plane containing said major axis, is waisted. The reinforcement member and waisted region may lie at the same axial position between the end regions.
The body of elastomeric material may be of a progressively decreasing external cross-sectional dimension over a third part of the length of the body extending from the first part to the reinforcement member.
Preferably the first part, or first and fourth parts, of progressively increasing diameter occupies at least 50% and more preferably between 60 and 90% of the length of the body.
One or each of the end regions may be bonded to a metal plate.
Preferably the body of elastomeric material is substantially hollow at least at the second end region. The part of the body of elastomeric material lying between the second end region and reinforcement member may have a maximum external dimension which is greater than that of the part of the body between the reinforcement member and the first end region. Thus the maximum outer dimension between the aforementioned second and fourth parts may be greater than that between the first and third parts. The reinforcement member may extend outwards to have an outer boundary which extends outwards from said major axis by a distance at least as great as the maximum external dimension of the body of elastomeric material. The outer boundary of the reinforcement member may extend outwards beyond the maximum external dimension of the elastomeric body by at least 25% of the length of the body. It may extend beyond by at least 50% of the length.
The reinforcement member may be selected in size so as always to extend outwards from the major axis to beyond the elastomeric body, or at least part of the body neighbouring the reinforcement member, when the elastomeric spring is subjected simultaneously to maximum shear and compression loads.
The inner boundary of the reinforcement member may lie radially closer to the major axis of the elastomeric body than the maximum radial extent of the hollow end region of the elastomeric body. The inner boundary may lie radially inwards relative to the maximum radial extent of a bonded area at the first end region of the elastomeric body.
Preferably the body of elastomeric material is substantially hollow over at least 50% and more preferably over at least 70% of the length of the spring. It may be substantially tubular. It may have a closed end region; a vent passage may extend axially through a closed end region to communicate with the hollow cavity.
Preferably the maximum external dimension of the spring as considered in a cross-sectional plane perpendicular to the major axis is at a position where the cross-section of the body is annular, i.e. at a hollow portion.
Preferably, as considered in a plane containing the major axis of the body, the external profile of the body is curved over the first, second and fourth parts and preferably also over any third if provided.
The body of elastomeric material as considered in a cross-sectional plane perpendicular to the major axis preferably is of a generally circular external profile, and also of generally circular internal profile where hollow thereby to result in a spring having substantially uniform load-deflection properties in all transverse directions perpendicular to the major axis. Where, however, non-uniform properties are sought either or both of said internal and external profiles may be non-circular, an elliptical profile being considered particularly useful for many applications.
The body of elastomeric material may comprise two or more substantially inextensible reinforcement members provided at positions spaced apart in said first direction.
An inextensible reinforcement member may be of flexible material, or it may be substantially rigid. It may be formed with a plurality of apertures and moulded elastomeric body material lying either side of the reinforcement member may interconnect through the apertures.
The spring may be used in series with an air spring and may have a through passage comprising the hollow region for the purpose of allowing air to flow to and from the air spring.
With the exception of the substantially inextensible reinforcement member(s) and any rigid end plates the spring per se and an installation incorporating the spring preferably is devoid of structure which restricts deformation of the body of elastomeric material in normal use of the spring.
Two or more of the bodies of elastomeric material may be arranged in series with one another. In this arrangement the end region of one spring may be formed of elastomeric material and said material may be contiguous with the elastomeric material of the end region of the next successive body. Alternatively, for example, a single rigid member may be provided between and bonded to the end faces of two successive bodies of elastomeric material or each of two confronting end regions may comprise a rigid member, means being provided for securing together the two rigid members.
When two bodies are arranged in series, preferably they are arranged with their first end regions in proximity in the case of bodies having first end regions which are closed.
Preferably the hollow portion of the body of elastomeric material defines at least in part a cavity which is at ambient pressure notwithstanding deflection of the spring. Thus when the second of the end regions comprises a rigid plate and the hollow does not extend to the other end, it is preferred that said plate be formed with an opening which is in communication with the hollow cavity thereby to permit the flow of air or other ambient medium to or from the cavity.
Preferably at least when in situ in a suspension the body of elastomeric material is arranged such that when undergoing compression in the first direction the external surface of the body over that part, the second part, which is of progressively decreasing external dimension progressively comes into contact with and is supported by a rigid support member. The rigid support member may be comprised by an outer region of an end plate bonded to the body of elastomeric material or may be independent of said plate.